That DMA in video memory is a freakish idea. Good luck to anyone trying to realise it. Snails are having aching bellies because of laughter.
There are many ways to realize direct memory access to NICK's memory.
The one would be to replace the onboard 64k with a dual port memory. It can be a SD-ram with interleaving, or a real dual port memory.
The other possibility to would be to take the behaviour-description from KYP's FPGA project, and make it as an external NICK. Because the external NICK would have it's 64K memory, the direct memory access is also possible without changing the EP board.
In that external NICK scenario it would be possible to extend the 64KB of video memory to 512K or more to have full PAL (720 x 576 pixels) resolution, which takes 720*576*1byte=405 KB memory.
With dual port memory it is also possible to share the whole 4 MB of the EP memory with the NICK.